Internal combustion engine exhaust emissions, and especially diesel engine exhaust emissions, have recently come under scrutiny with the advent of stricter regulations, both in the U.S. and abroad. While diesel engines are known to be more economical to run than spark-ignited engines, diesel engines inherently suffer disadvantages in the area of emissions. For example, in a diesel engine, fuel is injected during the compression stroke, as opposed to during the intake stroke in a spark-ignited engine. As a result, a diesel engine has less time to thoroughly mix the air and fuel before ignition occurs. The consequence is that diesel engine exhaust contains incompletely burned fuel known as particulate matter, or “soot”. In addition to particulate matter, internal combustion engines including diesel engines produce a number of combustion products including hydrocarbons (“HC”), carbon monoxide (“CO”), nitrogen oxides (“NOx”), and sulfur oxides (“SOx”). Aftertreatment systems may be utilized to reduce or eliminate emissions of these and other combustion products.
FIG. 1A shows a block diagram providing a brief overview of a vehicle powertrain. The components include an internal combustion engine 20 in flow communication with one or more selected components of an exhaust aftertreatment system 24. The exhaust aftertreatment system 24 optionally includes a catalyst system 96 upstream of a particulate filter 100. In the embodiment shown, the catalyst system 96 is a diesel oxidation catalyst (DOC) 96 coupled in flow communication to receive and treat exhaust from the engine 20. The DOC 96 is preferably a flow-through device that includes either a honeycomb-like or plate-like substrate. The substrate has a surface area that includes (e.g., is coated with) a catalyst. The catalyst can be an oxidation catalyst, which can include a precious metal catalyst, such as platinum or palladium, for rapid conversion of hydrocarbons, carbon monoxide, and nitric oxides in the engine exhaust gas into carbon dioxide, nitrogen, water, or NO2.
Once the exhaust has flowed through DOC 96, the DPF 100 is utilized to capture unwanted diesel particulate matter from the flow of exhaust gas exiting engine 20, by flowing exhaust across the walls of DPF channels. The diesel particulate matter includes sub-micron sized solid and liquid particles found in diesel exhaust. The DPF 100 can be manufactured from a variety of materials including but not limited to cordierite, silicon carbide, and/or other high temperature oxide ceramics.
The treated exhaust gases can then proceed through diesel exhaust fluid doser 102 for the introduction of a reductant, such as ammonia or a urea solution. The exhaust gases then flow to a selective catalytic reduction (SCR) system 104, which can include a catalytic core having a selective catalytic reduction catalyst (SCR catalyst) loaded thereon.
System 24 can include one or more sensors (not illustrated) associated with components of the system 24, such as one or more temperature sensors, NOx sensor, oxygen sensor, mass flow sensor, and a pressure sensor.
As discussed above, the exhaust aftertreatment system 24 includes a Selective Catalytic Reduction (SCR) system 104. The SCR system 104 includes a selective catalytic reduction catalyst which interacts with NOx gases to convert the NOx gases into N2 and water, in the presence of an ammonia reductant. The overall reactions of NOx reductions in an SCR are shown below.4NO+4NH3+O2→4N2+6H2O  (1)6NO2+8NH3→7N2+12H2O  (2)2NH3+NO+NO2→2N2+3H2O  (3)Where Equation (1) represents a standard SCR reaction and Equation (3) represents a fast SCR reaction.
The performance of the SCR catalyst is often counterbalanced by catalyst durability. This challenge is further compounded by the increasingly stringent emissions regulatory demands on the one hand, and the economic pressure surrounding fuel economy on the other. Furthermore, the performance of the SCR catalyst is influenced by the level of engine out NOx (EO NOx) that has to be processed by the SCR catalyst. The current trend is in the direction of higher engine out NOx to improve fuel economy, while emission levels are simultaneously being reduced. For example, at present, EO NOx can reach as high as 7 g/kW-hr for at least a short period of time. However, it is anticipated that in the future, there will be a move towards very low tailpipe NOx (e.g., decreasing from about 0.2 to about 0.02 g/kW-hr).
High EO NOx has been shown to result in urea deposit build up in the SCR, due to the extremely high levels of diesel exhaust fluid that is introduced into the system, and insufficient residence time for complete decomposition to form NH3. The formation and accumulation of urea deposits on the SCR catalyst can result in severe damage to both the chemical and physical integrity of the SCR coating. Furthermore, the high intensity of diesel exhaust fluid dosing and the relatively long duration of the dosing in urea decomposition reactor 102 can result in large quantities of water being released onto the SCR catalyst. As the SCR catalyst can be supported by zeolites, which are powerful water adsorbing materials, the quantities of water can present a problem with both durability and cold start performance of the SCR catalyst.
At low EO NOx conditions, challenges are similar to those present under extended idling and cold start conditions. In other words, when SCR temperatures are too low for diesel exhaust fluid dosing and normal SCR operation (between about 250-450° C.), other strategies are required to meet emissions standards.
These circumstances place SCR catalyst chemistry at the frontline of systems development for aftertreatment technology. As engine out NOx levels increases, diesel exhaust fluid (e.g., NH3) dosing must also increase to provide adequate amounts of NH3 reductant to meet emissions control requirements. However, at low engine out NOx the SCR's performance becomes more dependent upon storage capacity for NH3 and upon low temperature performance, rather than upon diesel exhaust fluid dosing and decomposition.
Thus, there is a need for a high durability SCR catalyst that is able to withstand the harsh environments resulting from high intensity diesel exhaust fluid dosing. The SCR catalyst should have the requisite emissions controls properties and relatively low hydrophilic properties. The SCR catalyst should exhibit an ability to rapidly release any absorbed water under cold start conditions. The SCR catalyst can function at low temperatures, based upon relatively high NH3 storage at low temperatures. The SCR catalyst should have good NOx storage ability to effectively remove NOx from the exhaust stream (temporarily), and then can enable the reduction of stored NOx to N2 at elevated temperatures. The SCR catalyst should be capable of operating in less than optimal NO2/NOx levels (i.e., <0.5), which is required for the fast SCR reaction. SCR catalysts with enhanced standard SCR reaction capability are also needed. The present disclosure seeks to fulfill these needs and provides further related advantages.